February 26th, 2017 by

Visiting the IRF, space instrument construction tips

Last week Tomas and Maria visited IRF, the Swedish Institute of Space Physics, in Kiruna. The reasons were two-fold: first the science part, which is mainly Maria’s area. The second reason was engineering, which is Tomas’ domain and the main focus of this post.

For the science bit, getting feedback from the space science community is important. It allows us to be reasonably confident that we know what problems expect when it comes to measuring. It also helps advertise our work to scientists that may be interested in our data, and gives ideas about possible future missions.

As for engineering, lots and lots of useful tips. In no particular order:

Cabling: wires should be bundled into harnesses. Pin headers + sockets are preferable, since they still allow some springiness but will not have much trouble with vibration. PTFE, Kapton and FEP are good, thermal vacuum compatible insulation materials. PVC is unacceptable. Flat flex is OK, but may crack or disconnect due to vibration.

Connectors: Every connector should be unique, so there is zero risk of incorrect hookup. Keep “protruding” connector elements for power on the instrument side, so that cables used for power have their elements “sunk in”. Typically this means female (sleeve) type of connector, but some pin types are also sunken. Use gold plated connectors.

Solder joins: Rosin core 63/37 SnPb solder has a predictable melting point and cleans well enough with isopropyl alcohol. Use clear Kynar heat shrink tubing where possible, especially on D-subminiature connectors, see picture below. This prevents loose pieces of wire from accidentally shorting pins

Proper soldering of DD-50 connector, with protective Kynar heat shrink tubing

Proper soldering of DD-50 connector, with protective Kynar heat shrink tubing

Cleaning: brush using pure isopropyl alcohol or 70/30 isopropyl alcohol/distilled water. The latter may be needed to remove certain salts. Rinse with pure isopropyl alcohol afterwards. DO NOT use ultrasonic cleaning if using any ceramic parts; they may crack.

Glue/epoxy: Scotch-Weld 2216 is thermal vacuum safe. Mix, then use vacuum to draw out any air trapped inside. After vacuuming, the mix is good for about 15 minutes. Use Scotch-Weld to hold down large ICs, spot glue bodge cables or anything else that might move but shouldn’t. Useful for gluing nuts, since locking nuts are not thermal vacuum safe.

PCB laminates: FR4 outgasses slightly, but is perfectly fine thermal vacuum-wise. High-voltage boards should use ceramic substrates such as Rogers R4003 since the outgassing of FR4 may create a rarefied atmosphere, causing arcing and equipment failure. This is not a problem for us luckily 🙂

Digital logic: short missions near Earth (such as ours) do not need any kind if special ICs. Mechanical structures that withstand vibration testing are thick enough to shield ICs from any problematic radiation. As an example, IRF have sent instruments using off-the-shelf ARM chips without problems.

Latch-up protection: a series resistor followed by a capacitor to ground is enough to protect chips from blowing up in case of latch-up. Power cycle to remove any latch-up situations.

Plastic parts: PEEK works well enough for most plastic parts, and is thermal vacuum safe.

Tape: usually Kapton is fine, but pay attention to the glue used. There are also tapes with specific thermal radiation properties which may be used in lieu of plating or painting.

These tips should be handy both for AMSAT folks and possibly anyone who’s looking into building vacuum equipment. Finally, I’m going to round this post off with some more pictures:

Thermal vacuum (TVAC) chamber

Thermal vacuum (TVAC) chamber

XSAN instrument being prepped for thermal vacuum test

XSAN instrument being prepped for thermal vacuum test

The *big* vacuum chamber

The big vacuum chamber

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